The violence that also left hundreds injured was the worst in nearly three months of anti-government protests that have paralyzed Kiev, Ukraine's capital. The opposition and President Viktor Yanukovych's government are locked in a stalemate over the identity of their nation of 46 million, which is divided in its loyalties between Russia and the West.
In an ominous development, Ukraine's top security agency accused protesters Wednesday of seizing hundreds of firearms from its offices and announced a nationwide anti-terrorist operation to restore order. The Defense Ministry said the army could take part in the operation.
Demonstrators, meanwhile, forced their way into the post office on Independence Square, also known as the Maidan, after a nearby building they had previously occupied was burned down Tuesday in fierce, fiery clashes with riot police. Thousands of activists armed with fire bombs and rocks had defended the square Tuesday, which has become a key symbol of the protests.
Black smoke from the sprawling, now-burnt opposition camp was still rising above the center of Kiev 24 hours later.
The shocking escalation of violence has prompted the European Union to threaten sanctions against Ukrainian officials responsible for the bloodshed and triggered an angry rebuke from Moscow, which accused the West of setting off the clashes by backing the opposition. The 28-nation EU is holding an emergency meeting on Ukraine in Brussels on Thursday.
In Paris, Foreign Minister Laurent Fabius told reporters Wednesday in a joint appearance with U.S. Secretary of State John Kerry that he and his counterparts from Germany and Poland would meet both sides in Ukraine ahead of the EU meeting on possible sanctions.
He said he hoped the two sides "will find a way for dialogue."
Possible sanctions include banning leading officials from traveling to EU nations or freezing their assets there. Travel bans and assets freezes for the powerful oligarchs who back Yanukovych could prompt them to pressure him to change course.
But the bad blood in Ukraine now runs so high it has fueled fears the nation could be sliding toward a messy breakup. While most people in the country's western regions resent Yanukovych, he enjoys strong support in the mostly Russian-speaking eastern and southern regions, where many want strong ties with Russia.
Neither side now appears willing to compromise, with the opposition insisting on Yanukovych's resignation and early elections and the president prepared to fight till the end.
Amid a stagnating economy and soaring corruption, Ukrainians have been angered to see that Yanukovych's close friends and family have risen to top government posts and amassed fortunes since he came to power in 2010. Yanukovych's son, Oleksander, a dentist, has become a financial and construction magnate worth $187 million, according to Forbes Ukraine.
Yanukovych on Wednesday blamed the protesters for the violence and said the opposition leaders "crossed a line when they called people to arms."
"I again call on the leaders of the opposition ... to draw a boundary between themselves and radical forces, which are provoking bloodshed and clashes with the security services," the president said in a statement. "If they don't want to leave (the square) - they should acknowledge that they are supporting radicals. Then the conversation with them will already be of a different kind."
He also called for a day of mourning Thursday for the dead.
Radical protesters willing to confront police with violence were largely shunned at the start of the peaceful demonstrations three months ago, but they have become a key force in recent weeks, with moderate demonstrators bringing them food and even preparing Molotov cocktails for them. Police also have turned increasingly brutal after law enforcement officers were killed.
Opposition lawmaker Oleh Lyashko warned that Yanukovych himself was in danger if he does not offer some concessions.
"Yanukovych, you will end like (Moammar) Gadhafi," Lyashko told thousands of angry protesters. "Either you, a parasite, will stop killing people or this fate will await you. Remember this, dictator!"
The protests began in late November after Yanukovych turned away from a long-anticipated deal with the EU in exchange for a $15 billion bailout from Russia. The political maneuvering has continued since, with both Moscow and the West eager to gain influence over this former Soviet republic.
The Kremlin said it put the next disbursement of its bailout on hold amid uncertainty over Ukraine's future and what it described as a "coup attempt."
European Commission President Jose Manuel Barroso, meanwhile, called for "targeted measures against those responsible for violence."
"It was with shock and utter dismay that we have been watching developments over the last 24 hours in Ukraine," he said.
The latest bout of street violence began Tuesday when protesters attacked police lines and set fires outside parliament, accusing Yanukovych of ignoring their demands to enact constitutional reforms that would limit the president's power - a key opposition demand. Parliament, dominated by his supporters, was stalling on taking up a constitutional reform to limit presidential powers.
Police responded by attacking the protest camp. Armed with water cannons, stun grenades and rubber bullets, police dismantled some barricades. But the protesters held their ground through the night, encircling the protest camp with new burning barricades of tires, furniture and debris.
On Wednesday morning, the center of Kiev was cordoned off by police, the subway was shut down and most shops on Kiev's main street were closed. But hundreds of Ukrainians still flocked to the opposition camp, some wearing balaclavas and armed with bats.
One group of young men and women poured petrol into plastic bottles, preparing fire bombs, while a volunteer walked by distributing ham sandwiches. Other activists were busy crushing the pavement into bags to fortify the barricades.
"The revolution has turned into a war with the authorities," said Vasyl Oleksenko, 57, a retired geologist from central Ukraine who fled Tuesday night's violence fearing for his life. "We must fight this bloody, criminal leadership. We must fight for our country, our Ukraine."
The Health Ministry said 25 people died in the clashes, some from gunshot wounds, and Kiev hospitals were struggling to treat hundreds of injured.
Meanwhile, in the western Ukrainian city of Lviv, where most residents yearn for stronger ties with the EU and have little sympathy for Yanukovych, protesters seized several government buildings, including the governor's office, police stations, and offices for prosecutors, security officials and the tax agency. They also broke into an Interior Ministry unit and set it on fire.
In another western city, Lutsk, protesters handcuffed the regional governor, a Yanukovych appointee, and tied him to the stage on a central square after he refused to resign.
In the city of Khmelnitsky, three people were injured when protesters tried to storm a law enforcement office.
Government buildings were stormed or besieged in other western cities.
Ukraine's ailing economy is a major factor in the crisis. On Monday, Russia said it was ready to resume providing the loans that Yanukovych's government needs to keep the country afloat. This raised fears among the opposition that Yanukovych had made a deal with Moscow to stand firm against the protesters and would choose a Russian-leaning loyalist to be his new prime minister.
President Vladimir Putin's spokesman, Dmitry Peskov, said in a statement that Putin spoke to Yanukovych overnight by phone but that it's up to the Ukrainian government to settle its own problems.
Peskov also added that the next Russian bailout payment is on hold, saying the priority now is to settle the crisis, which he described as a "coup attempt."
The Russian Foreign Ministry issued a statement blaming the West for failing to condemn the opposition for the latest bout of violence, while EU leaders took the opposite stance.
"Today, President Yanukovych has blood on his hands," Swedish Foreign Minister Carl Bildt said.
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